This application claims priority from Japanese Patent Application No. 11-016141 filed Jan. 25, 1999, the contents of which are incorporated hereinto by this reference.
1. Field of the Invention
The present invention relates to a positive electrode for a non-aqueous electrolyte cell containing spinel-type lithium manganese oxide as a primary active material and a manufacturing method of the same. The present invention is also concerned with a non-aqueous electrolyte cell including a negative electrode comprised of a negative active material capable of intercalating and deintercalating a lithium ion, a positive electrode containing spinel-type lithium manganese oxide and a non-aqueous electrolytic material and a manufacturing method of the non-aqueous electrolyte cell.
2. Description of the Prior Art
In recent years, a non-aqueous electrolyte cell such as a lithium-ion cell has been commercially used as a rechargeable cell of small size, light weight, higher capacity for portable electronic devices and communication equipments such as a small size video camera, a portable telephone, a note-type personal computer, etc. The non-aqueous electrolyte cell includes a negative electrode active material in the form of an alloy or a carbon material capable of intercalating and deintercalating a lithium-ion and a positive electrode material in the form of a transition metal oxide containing lithium such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide or the like.
In the transition metal oxide containing lithium for the non-aqueous electrolyte cell, lithium nickel oxide (LiNiO2) is useful to provide a higher capacity cell but inferior in stability and function compared to lithium cobalt oxide (LiCoO2). Lithium manganese oxide (LiMn2O4) is rich in resources and obtainable at a low cost but inferior compared to lithium cobalt oxide (LiCoO2) since it is lower in energy density and the manganese itself is dissolved at a high temperature. For these reasons, lithium cobalt oxide is mainly used as the transition metal oxide containing lithium.
However, when a non-aqueous electrolyte cell of this kind is useful in large size equipments such as hybrid automotive vehicles, it is sometimes decided to use lithium manganese oxide (LiMn2O4) which is rich in resources and lower in price instead of lithium cobalt oxide (LiCoO2). Under such technical background, as in Japanese Patent Laid-open Publication No. 9-293538, there has been proposed a method for enhancing the energy density of lithium manganese oxide by addition of lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2).
The method disclosed in Japanese Patent Laid-open Publication No. 9-293538 is, however, insufficient for enhancement of the energy density of lithium manganese oxide. To enhance the property of lithium manganese oxide used as an active material for a positive electrode, it is very important to enhance its high temperature cycle performance and storage characteristics. As to its high temperature performance, there have been proposed various methods for stabilizing the crystal structure of the active material by addition of different kinds of elements. However, an effective substitution element such as chromium is injurious to humans, and the energy density of the active material is decreased if the amount of additive elements is excessive. For these reasons, there has not yet been proposed any practical method for improvement of its high temperature cycle performance.
As to storage characteristics of the active material, self-discharge of lithium manganese oxide (LiMn2O4) caused by reaction with electrolyte results in generation of gaseous products. This deteriorates cell characteristics. Such phenomena will noticeably occur if the lithium manganese oxide is stored in a discharging condition. Additionally, when the lithium manganese oxide is preserved at a high temperature, a large amount of gaseous products will generate due to dissolution of manganese itself.
A primary object of the present invention is to provide a positive electrode of higher energy density for a non-aqueous electrolyte cell in which self-discharge of lithium manganese oxide used as a main active material is restrained to enhance storage characteristics of the cell at higher temperature and to provide a non-aqueous electrolyte cell superior in the characteristics described above.
According to the present invention, the object is accomplished by providing a positive electrode for a non-aqueous electrolyte cell comprised of a mixture of spinel-type lithium manganese oxide represented by a general formula Li1+xMn2xe2x88x92YO4 (provided that an atomic ratio of lithium and manganese is defined to be 0.56xe2x89xa6Li/Mn [=1+X)/(2xe2x88x92Y)]xe2x89xa60.62 where X and Y are defined to be xe2x88x920.2xe2x89xa6Xxe2x89xa60.2 and Yxe2x89xa61.0, respectively) and lithium cobalt oxide represented by a general formula Li1+ZCoO2 (xe2x88x920.5xe2x89xa6Zxe2x89xa60.5) or lithium nickel oxide represented by a general formula Li1+ZNiO2(xe2x88x920.5xe2x89xa6Zxe2x89xa60.5).
Lithium manganese oxide represented by the formula acts as a strong oxidization agent and generates a large amount of gaseous products due to reaction with electrolytes and electrolyte salts. This deteriorates the performance of the cell and deforms the cell due to abnormality in internal pressure, resulting in leakage of the electrolyte. However, in the case that the positive electrode is comprised of the mixture of lithium manganese oxide and lithium cobalt oxide or lithium nickel oxide, it has been found that that the generation amount of gaseous products is noticeably decreased since lithium cobalt oxide or lithium nickel oxide acts as a buffer agent to restrain oxidation of the electrolyte caused by lithium manganese oxide.
When a positive electrode of lithium manganese oxide is stored in a discharging condition, the electrolyte is decomposed by self-discharge of the electrode and generates a large amount of gaseous products. In such an instance, reaction of the discharged lithium manganese oxide with the electrolyte decreases the cell voltage and increases the negative electrode voltage. This causes the generation of a large amount of decomposed gaseous products of the electrolyte. However, in the case that lithium cobalt oxide or lithium nickel oxide is nixed with lithium manganese oxide to provide the positive electrode, it has been found that the generation of decomposed gaseous products is restrained even when the positive electrode is stored in a discharging condition.
In this case, it is assumed that when lithium manganese oxide becomes unstable at a final stage of discharge, lithium cobalt oxide or lithium nickel oxide becomes effective to prevent deterioration of lithium manganese oxide for enhancing the storage characteristics of the cell. It is also assumed that lithium cobalt oxide or lithium nickel oxide acts as a buffer agent to restrain dissolution of manganese for restraining the generation of gaseous products.
On the other hand, a retention rate of discharge capacity at the high temperature discharge cycle was measured by an experiment in relation to the atomic ratio of lithium and manganese in the spinal-type lithium manganese oxide. As a result, it has been found that the retention rate of discharge capacity is optimized when the atomic ratio of lithium and manganese is determined equal to or more than 0.56. In addition, the capacity per a unit active material (specific capacity) was measured by experiments in relation to the atomic ratio of lithium and manganese. As a result, it has been found that the specific capacity is maximized when the atomic ratio of lithium and manganese is defined to be equal to or less than 0.62. From these facts, it is desirable that the atomic ratio Li/Mn of lithium and manganese in spinel-type lithium manganese oxide is defined to be 0.56xe2x89xa6Li/Mnxe2x89xa60.62.
The effect of lithium cobalt oxide or lithium nickel oxide as the buffer agent increases in accordance with an increase of the mixed amount thereof. It is, therefore, desirable that the addition amount of lithium cobalt oxide and/or lithium nickel oxide is determined to be equal to or more than 5% by weight. In general, the discharge voltage of lithium cobalt oxide or lithium nickel oxide is low. It is, therefore, assumed that the addition of lithium cobalt oxide or lithium nickel oxide is effective to decrease the discharge voltage of the cell lower than that of lithium manganese oxide. However, as lithium cobalt oxide or lithium nickel oxide is superior in electronic conductivity, the discharge voltage of the cell was increased by addition of lithium cobalt oxide or lithium nickel oxide.
In the experiments, it has been found that if the addition amount of lithium cobalt oxide or lithium nickel oxide is more than 20% by weight, the discharge voltage of the cell is greatly influenced by lithium cobalt oxide or lithiun nickel oxide. It is, therefore, desirable that the addition amount of lithium cobalt oxide or lithium nickel oxide is less than 20% by weight. In the case that the weight of spinel-type lithium manganese oxide is defined as A and that the weight of lithium cobalt oxide or lithium nickel oxide is defined as B, it is desirable that the amount of lithium cobalt oxide or lithium nickel oxide mixed with lithium manganese oxide is determined to be 0.05xe2x89xa6B/(A+B) less than 0.2.
To improve the charge-discharge cycle performance of lithium manganese oxide, lithium cobalt oxide and lithium nickel oxide at a high temperature, various researches have been carried out to stabilize a crystal structure of the elements by addition of different kinds of metals. However, any practical different kind of metal has not yet been found. As a result of investigation of different kinds of metals to be added to a positive electrode containing a mixture of lithium manganese oxide with lithium cobalt oxide or lithium nickel oxide, it is has been found that a practical metal can be selected from the group consisting of magnesium, aluminum, calcium, vanadium, titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, molybdenum and tin.
In the case that the different kind of metal is added to spinel-type lithium manganese oxide, it is desirable that the atomic ratio of lithium relative to a sum of manganese and the different metal is determined to be 0.56xe2x89xa6Li less than (Mn+Different metal)xe2x89xa60.62 to increase the retention rate of discharge capacity at the charge-discharge cycle at a high temperature and to increase the specific capacity per unit active material.
In the case that lithium cobalt oxide or lithium nickel oxide is added to and mixed with lithium manganese oxide to produce the positive electrode, a degree of direct contact of particles is decreased if the lithium materials are simply mixed. In such an instance, the additive effect of the lithium cobalt oxide or lithium nickel oxide becomes insufficient. To increase the degree of direct contact of particles, it is desirable that the lithium materials are mixed under compression and crushing action or under compression, impact and shearing actions. In the case that the lithium materials are mixed under compression and crushing action, secondary particles of the materials are damaged, resulting in deterioration of the performance of the positive electrode. In contrast with the above case, in the case that the lithium materials are mixed under compression, impact and shearing actions, secondary particles of the materials are maintained without any damage to enhance the additive effect of lithium cobalt oxide or lithium nickel oxide.
The positive electrode manufactured by addition of lithium cobalt oxide or lithium nickel oxide to lithium manganese oxide can be adapted not only to a non-aqueous electrolyte cell using organic electrolyte but also to a non-aqueous electrolyte cell using solid polymer electrolyte. As the solid polymer electrolyte is relatively higher in viscosity, a problem in impregnation of the electrolyte will occur in use of only lithium manganese oxide. In this respect, the positive electrode manufactured by addition of lithium cobalt oxide or lithium nickel oxide to lithium manganese oxide can be formed thinner to solve the problem in impregnation of the electrolyte. In this case, it is preferable that the solid polymer electrolyte is used in the form of mixed gel of lithium salt, electrolyte, and polymer selected from the group consisting of solid polymer of polycarbonate, solid polymer of polyacrylonitrile, copolymer or bridged polymer comprised of more than two kinds of the solid polymers and flouride solid polymer such as polyvinylidene difluoride (PVDF).